Attempts were made to assess etiologic factors for a variety of hormonally related tumors, including cancers of the breast, ovary, endometrium and prostate. An assessment of pre- and post-natal characteristics showed elevations in breast cancer risk among twins, particularly dizygotics, consistent with the hypothesis that high levels of estrogen in utero predispose to the disease. Subjects who were breast fed as infants were at a low risk of subsequent breast cancer. Women who experienced nausea or vomiting during pregnancy appeared to be at a reduced risk of breast cancer. A variety of other medical conditions, however, did not appear to be related to risk. In contrast to some other investigations, high levels of physical activity, either over life or during adolescence, did not affect breast cancer risk. Studies were also undertaken to assess reasons for the racial variation in breast cancer incidence, with attempts made to understand why young (<40 years) black women have high rates of breast cancer, a trend reversed at older ages. It was unclear why young black women had high rates of disease, but the racial difference among older women appeared primarily due to whites having later ages at first birth and fewer children than blacks. Given previous findings that obesity relationships with endometrial cancer did not appear to be explained by endogenous hormones (including estrogens), we assessed risk in relation to other biomarkers, but found no relationship of risk with insulin levels. Follow up of a large cohort of women hospitalized for endometriosis showed elevated risks of ovarian and breast cancers, as well as non-Hodgkin's lymphoma, indicating the need for further studies to assess hormonal and immunologic hypotheses. In a case-control study of prostate cancer, undertaken in a low-incidence area (China), risk appeared related to body size, calories, and consumption of red meat. Future analyses in this study will focus on relationships of risk with fatty acids and endogenous hormones, as elaborated upon in two recently prepared editorials. A variety of methodologic studies were also undertaken to assess the reproducibility of both serum and urine hormones. These studies will guide us in determining which endogenous hormones should be assessed in future analytic studies of breast and prostate cancers.